Defining the orientation of the IMU

Paparazzi has a parameter to define the orientation of the IMU with respect to the body of the vehicle.

Important Not all algorithms properly deal with any BODY_TO_IMU orientation. See the notes for the respective implementation at Subsystem/ahrs and the issue on github.
Currently the only way to properly deal with 90deg rotations is to just change the sensors channels (e.g. so that what are normally the x-axis gyros and accels become the y-axis sensors, etc.)